It is known that, in Diesel engines, the ignition of the fuel is achieved by increasing the temperature of the charge of air, which increase is due to the adiabatic compression of the air in the cylinders. This is why the volumetric compression ratio, usual in these engines, is relatively high and generally not less than 12.
It is also known that the horse power of Diesel engines may be increased by supercharging, generally by means of a turbo-supercharger, the charge of air introduced into the cylinders, thus increasing the pressure at the end of compression and, consequently, the maximum pressure in cycle. The rate of supercharging is commonly from 2 to 3.
However, in order to limit the maximum pressures in cycle at full load, the volumetric compression ratio of supercharged engines is generally fixed at the minimum value compatible with good starting of the engine. It follows that in operation at no-load or at low-load, the temperatures in cycle are low, and on prolonged operation in cold weather the lubricating oil, set up again by the demulsification, cannot burn completely and "sticks" the exhaust valves. Likewise, during the cold weather, starting may be difficult as the ignition temperatures are difficult to reach.